1. Field of the Invention
The present invention relates to apparatus for measuring velocity, and, more particularly, for measuring velocity in the range of interest in seismic exploration.
2. Description of the Prior Art
In seismic exploration, seismic waves are commonly used to probe the earth""s crust as a means of determining the type and location of subsurface formations. The earth""s crust can be considered a transmission medium or filter whose characteristics are to be determined by passing seismic waves through that medium. In the reflection seismic method, seismic waves are impulses are generated at a point at or near the earth""s surface, and the compressional mode of those waves is reflected from subsurface acoustic impedance boundaries and detected by arrays of seismic detectors located at the surface of the earth. The seismic detectors convert the received waves into electrical signals which are sensed and recorded in a form which permits analysis. Skilled interpreters can discern from such an analysis the shape and depth of subsurface reflection boundaries, and the likelihood of finding an accumulation of minerals, such as oil and gas.
One type of seismic detector which has been utilized is a device having a proof mass suspended by one or more springs within a sensor housing such that the proof mass is limited in motion along the central axis of the housing. Such axial motion of the proof mass is opposed by the springs and is indicative of the velocity along the central axis. Such prior art devices are subject to variations in the spring length and in the modulus of elasticity of the springs due to temperature variations. Additionally, restraint of the proof mass from movement other than axially has tended to generate mechanical noise and erroneous measurements, as discussed in U.S. Pat. No. 5,756,896.
Other prior art in the field of the measurement of velocity has included the utilization of a magnetic or a permeable proof mass suspended in a ferro fluid within a sensor housing. The movement and positioning of the proof mass within the sensor housing is controlled by and often measured by variations in the magnetic field strength of an internal or external magnetic field. All of the known prior art apparatus which utilizes an external electromagnetic field to control the movement or positioning of the proof mass have a limit on sensitivity resulting from the noise inherent in the electrical/electronic circuits driving and/or comprising the electromagnetic field generator.
U.S. Pat. No. 5,756,896, which is incorporated herein by reference, discloses apparatus for measuring velocity over a wide range of values, which apparatus does not introduce noise into its proof mass detecting means. While the velocity sensor disclosed in the ""896 patent was a substantial improvement over existing velocity sensors, certain characteristics of that velocity sensor, e.g., resonant frequency, tend to shift over time.
In accordance with the present invention, a velocity sensor having enhanced stability characteristics is provided. An embodiment of a velocity sensor in accordance with the present invention includes a housing and a magnetic proof mass suspended in the housing in a ferromagnetic fluid. The magnetic proof mass has a cylindrical central portion and end portions which are uniformly tapered from the central portion to the ends of the magnetic proof mass. A magnetic proof mass having this configuration has the a strongest magnetic field strength in the area of least diameter of the proof mass (i.e., the end of the tapered portion), which enhances the stability of the resonant frequency of the velocity sensor. Further, the magnetic proof mass in a velocity sensor according to the present invention has rounded ends, since magnetic ends with sharp edges tend to concentrate the magnetic fields near the sharp edges and thus, the effective fluid density in the area of the sharp edges.
In accordance with the present invention, it has also been found that stabilization of the resonant frequency of the velocity sensor may be enhanced by plating a portion of the central portion of the magnetic proof mass with a paramagnetic (i.e., weakly magnetic) coating to provide a minimum, constant shunt path for the magnetic particles in the ferromagnetic fluid.
A velocity sensor in accordance with the present invention also includes sharp edged caps, or rings, which are made of a non-magnetic material and which are mounted on the magnetic proof mass to ensure that the fluid flow around the magnetic proof mass is the same in all radial positions. These caps or rings function to minimize the distortion in the output of the velocity sensor.
The ferromagnetic fluid which is used in a velocity sensor according to the present invention comprises very fine magnetic particles which are coated with a specialized surfactant to promote colloidal suspension within a hydrocarbon solvent. An aggressive, non-hydrocarbon solvent is added to the ferromagnetic fluid in a relatively large quantity to keep the molecules of the surfactant spread apart at low temperatures. A commercially available pour point depressor is also added to the ferromagnetic fluid in very low quantities to compensate for the change in hydrocarbon solvent.